Method of continuous casting an alloy having a two phase region during cooling



Feb. 17,1910

Filed Sept. 19. 1966 TEMPERATURE C 1'. D. WATERS ETAL 3,495,649 METHOD- OF CONTINUOUS CASTING AN ALLOY HAVING N DURING COOLING A TWO PHASE REGIO 3 sheets Sheet l MIN/MUM CASTING TEMPERATURE I I l L/OUID 7000 L/OU/D 1+ L/OU/D 2 PRIMARY SOL lD/F/CAT/ON B\ TEMPERATQRE SOL/096+ L/0U/D2 SECONDARY SOLID/FICAT/ON c ,TEMPERATURE SOL/D06 sou ""10"'2; '3'0' '4?) WEIGHT PERCENTAGE LEAD Luvswrozs THOMAS D-WAIEES- AT TO 2 5Y5 Feb. 17, 1970 T. D. WATERS ETAL 3,495,649

' METHOD OF CONTINUOUS CASTING AN ALLOY HAVING A TWO PHASE REGION DURING COOLING Filed Sept. 19, 1966 5 Sheets-Sheet 2 v1 4 g l s &

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INVENToRs THO-MAS D..WATERS Mmlu E. MALAM A-uzewE.EosEa1-s u Y PM, H MM,

' ATTORNIEYE,

Feb. 17, 1970 1'. D. WATERS ETAL 3,495,649

METHOD OF CONTINUOUS CASTING AN ALLOY HAVING A TWO PHASE REGION DURING COOLING Filed sept. 19, 1966 5 Sheets-Sheet 3 I NVENTOES THOMAS QWATE25 Mmw E. MALAM U I I Y A'NDEEW ERosErrsou United States Patent Office 3,495,649 Patented Feb. 17, 1970 US. Cl. 164-82 12 Claims ABSTRACT OF THE DISCLOSURE Continuously casting an alloy which exhibits, on cooling, a transition from a single homogeneous phase to a two phase liquid region to be cast, the casting is commenced in a vertical direction at a temperature high enough for the alloy to remain as a homogeneous liquid phase with a minimum casting temperature isothermal lying substantially within the upstream end of a casting die.

This invention relates to a method of casting, by continuous or semi continuous means, alloys of the kind that exhibit on cooling a transition from a single homogeneous phase to a two phase liquid region to be cast, with a uniform or controlled non-uniform dispersion of one phase within the other in the solid state. There is disclosed an article of manufacture produced by the method and in particular but not exclusively to plain bearing material from which a bearing can be fabricated.

The term plain bearing is to be understood as including any member or assembly having, or designed to have in use, a surface which bears directly or through a liquid or solid lubricant against another surface relatively to which it has sliding movement, irrespective of whether the main or sole purpose is to transmit a load from one to the other of the surfaces having relative sliding movement or whether the sliding contact is solely or partly for some other purpose such, for example, as to provide a seal or to make electrical contact.

Alloys of the kind defined for example aluminium/ lead and aluminium/cadmium, are particularly advantageous as plain bearing materials. The term lead used herein is to be understood as including not only lead in substantially pure state but also lead alloys embodying for exexample, tin, indium, antimony or other alloying elements giving to the lead corrosion resisting or other desirable characteristics. The lead may contain up to 20% of its weight of tin or antimony or up to 10% of its weight of indium.

The term aluminium used herein is to be understood as including not only substantially pure aluminium but also aluminium with the impurities normally found in commercial aluminium, as well as aluminium alloys containing alloying elements such as copper, nickel, manganese, silicon, magnesium or the like for strengthening purposes.

The term minimum casting temperature, when used in this specification refers to the minimum temperature at which the alloy under equilibrium cooling conditions exhibits a single homogeneous liquid phase. Further reduction in temperature below the minimum casting temperature under equilibrium cooling conditions, produces two immiscible liquid phases.

The minimum casting temperature isothermal indicates a boundary in the molten metal at which transition from the single homogeneous phase to a double immiscible phase condition is commencing.

The major difiiculty in casting alloys of the kind defined above with a uniform dispersion of one phase within the other in the solid state is in overcoming or reducing the effectsof their tendency to gross gravity segregation on cooling through the two liquid phase temperature range.

The term gravity segregation" as used herein refers to a condition of non-uniform dispersion (on a microscopic scale) of one phase within the other in the solid state resulting from relative movement of said liquid phases to one another in a vertical direction under the influence of gravitational forces.

It is an object of the invention to nullify or control the effect of segregation so that its effect on the cast material is negligible, and the material is a fine and regular dispersion of one constituent within the other or a controlled non-uniform dispersion.

According to the present invention a method of continuously casting alloys of the kind defined includes commencing to cast the alloy in a vertical direction at a temperature high enough for the alloy to remain as a homogeneous liquid phase and casting under controlled casting conditions so that the minimum casting temperature isothermal as hereinbefore defined lies substantially within the upstream end of a die, whereby an approach to a uniform or controlled non-uniform dispersion of one phase within the other is obtained.

The temperature before commencing casting may be such that the alloy remains as a single homogeneous liquid phase in a feeder or tundish and in this case the minimum casting temperature isothermal will not extend into the tundish but will lie substantially wholly in the mouth of an opening from the tundish (or in the mouth of the tundish when the tundish mouth and the dies have the same internal dimensions) whereby the effects of gravity segregation are substantially reduced.

If the alloy is aluminium lead having 20% by weight of lead the tundish temperature may be maintained at not less than ll00 C. For an aluminium lead alloy having 10% by weight of lead the temperature in the tundish may be maintained at not less than 950 C. Whether with or without the tundish the method of continuous casting may be vertical or horizontal and the alloy may be cast directly onto a bearing backing mate rial.

The temperature of the alloy in the die may be reduced by the order of 450 C. for a 20% by weight lead-aluminium alloy and by the order of 300 C. for a 10% by weight lead aluminium alloy. On emergence from the die the alloy may be partially or wholly solid.

The die may be of graphite and of sufficient length in the direction of casting as to enable heat extraction to cause solidification of the alloy.

An apparatus is therefore disclosed for the continuous casting of aluminium lead including a tundish provided with heating means for maintaining aluminium lead above the minimum temperature at which it exists as a homogeneous liquid, a die opening from the tundish and cooling means surrounding the die. The temperature of the alloy is reduced while it is in the die by a temperature of the order of l5%50% of the minimum casting temperature while the minimum casting temperature isothermal lies wholly within the upstream opening into the die from the tundish.

A further aspect of the invention provides an aluminium lead bearing material having up to 50% by weight of lead produced by a continuous or semi-continuous casting process as defined, or in apparatus as defined.

The apparatus may be a horizontal casting apparatus or a vertical casting apparatus and in either case the casting of aluminium lead may be carried out directly onto a metal backing so as to provide a strip of material from which bimetallic bearings can be fabricated.

It is probable that a non-uniform distribution of a secondary phase within a primary phase may in some instances be desirable and control over the distribution of this secondary phase in a lateral plane is possible by varying the shape of the die and/ or tundish and their position relative to the minimum casting temperature isothermal. Thus by varying the projected area of the plane at which the minimum casting temperature isothermal exists, in relation to the projected area of the die at the primary solidification front, it is possible to control closely the distribution of the secondary phase within the primary phase in a non-uniform manner.

The invention will now be described by way of example with reference to the accompanying diagrammatic drawings in which:

FIGURE 1 shows a graph correlating the minimum casting temperature and the weight percentage of lead in the aluminium;

FIGURE 2 shows a diagram of an apparatus for carrying out the method according to the invention in connection with vertical casting;

FIGURE 3 shows a diagram for an apparatus for carrying out the invention in connection with horizontal casting direct onto a meal backing strip;

FIG. 4A, FIG. 4B, FIG. 4C, FIG. 4D, and FIG. 4B show various shapes or die which produce difference forms of phase distribution in the cast product; and

FIGURE 5 shows a casting apparatus for an alloy in which the constituent of higher specific gravity predominates over the constituent of lower specific gravity.

As shown in the graph in FIGURE 1, provided the temperature at which the molten alloy is maintained is above a curve A, aluminium and lead will exist as a homogeneous single liquid phase. The curve A is the minimum casting temperature curve plotted for various percentages of lead in the alloy.

In fact, the actual casting temperatures or the temperature at which the alloy is maintained in the tundish should be approximately 40 C. above the minimum casting temperature in order to ensure that fluctuations in operating conditions do not move the minimum casting temperature isothermal beyond the position limits.

The primary solidification temperature at which solidification commences is shown at B and the secondary solidification temperature at which solidification is complete is shown at C. The lines shown in FIGURE 1 represent alloy with more aluminium than lead.

On cooling the homogeneous aluminium-rich phase below the minimum casting temperature a lead-rich phase will commence to separate out under equilibrium cooling conditions. This is due to a progressive decrease in the solubility of lead in aluminium with decrease in temperature. Hence with continued decrease in metal temperature progressively more lead is rejected from the primary liquid phase in the form of a liquid lead-rich phase until the eutectic temperature (primary solidification temperature) of 685.5 C. is reached. At this temperature the primary aluminium-rich phase contains 1.52% by weight of lead and solidification of this phase occurs Without further decrease in temperature. Thus at this stage of solidification a solid aluminium-rich phase is in equilibrium with a liquid lead-rich phase. With continued lowering in metal temperature from the primary solidification temperature no major changes occur until on reaching the secondary solidification temperature of 326 C. the lead-rich phase completely solidifies at constant temperature. At this stage the alloy is completely solid.

FIGURE 2 shows a schematic representation of a method of operation according to the invention. The tundish 1 contains the alloy as a homogeneous single phase. The temperature of the metal in the tundish 1 can be maintained at the required level, either by superheating the alloy prior to transfer to the tundish, or by external heating of the tundish itself (neither super heat ing nor external heating arrangements are shown). While the alloy is maintained as a single homogeneous phase within the tundish 1 i.e. at a temperature greater than the minimum casting temperature there is no tendency for the secondary lead-rich liquid phase to separate out. On passage of the metal into the die 3 via the opening 2 from the tundish 1 the rate of heat extraction, speed of casting, and length of die 3 are regulated such that the minimum casting temperature isothermal A is maintained wholly within the confines of the die and the primary solidification from B is so disposed relative to the lower end of the die that the cast product can be satisfactorily withdrawn. Subsidiary water sprays (not shown) may be employed to facilitate the requisite heat extraction from the cast product.

When the alloy passes below the minimum casting temperature isothermal the progressive precipitation of the lead-rich phase previously described occurs with three major factors influencing the relative distribution of the two phases. Firstly, equilibrium cooling conditions may not be attained when continuously casting: secondly, the lead-rich phase once precipitated will fall through the less dense aluminium-rich liquid phase under the influence of gravitational forces; and finally the geometry of the die and friction between the molten metal and the die wall will interfere with the flow pattern. The maximum rate of fall of the lead-rich phase through the aluminium-rich phase depends on the size and shape of the lead particles and also the total time interval between the initial precipitation of those particles and their subsequent incorporation in the solidifying aluminiumrich phase at the primary solidification front. The relative velocity between the phases increases up to a maximum with increase in time after initial precipitation, and with increase in size of the precipitate. (The maximum relative velocity between phases may not be achieved in practice, as this depends on the secondary phase reaching its terminal velocity.)

On descending through the die 3 from the minimum casting temperature isothermal to the primary solidification front, a progressive increase in the amount of precipitated lead, and also the velocity of these lead-rich globules takes place. Thus, considering a narrow isothermal band across the die, lead in the form of globules is being lost to a similar isothermal band immediately below and being gained from another such band, immediately above. However, the amount of lead being lost is marginally greater than that being gained in any such isothermal band, because the amount and velocity of precipitated lead globules increases on descending through the die. An isothermal band immediately adjacent to, but below, the minimum casting temperature isothermal is only losing lead, and an isothermal band immediately adjacent to, but above, the primary solidification front is only gaining lead.

Hence, when the alloy passes from above the minimum casting temperature isotherma to below the primary solidification front, the majority of the lead is precipitated as a lead-rich liquid-phase, but the overall lead content is unchanged.

On cooling from the primary solidification front to the secondary solidification front, at which both faces are solid, no relative movement between the lead-rich and aluminium-rich phases is possible, as the latter phase is solid.

The extent of the lead-rich phase formation in the region D, FIGURE 2, and its attendant segregation under gravity is time/temperature dependant. The greater the temperature gradient (i.e. the smaller the distance D) existing between the primary solidification front B and the minimum casting temperature isothermal A the less will be the extent of the lead-rich phase formation and hence gravity segregation.

At the primary solification front there will be a tendency for precipitated lead particles to move under gravity along the solidification front to a position of lowest potential energy within the system. Thus, ideally, the primary solidification front should be horizontal to prevent segregation occurring by this method. This condition is only obtainable if uni-directional cooling exists. In general practice heat extraction is not unidirection and a dish-shaped solidification front rather than a horizontal front is obtained.

An approach to a horizontal solidification front rather than a dish-shaped front can be obtained b controlling the following factors. To reducevsegregation due to a dish-shaped solidification front, the rate of heat extraction from any point along the primary solidification front should be substantially the same. These conditions are approached by casting a billet which is thin in at least one direction at a relatively slow casting rate. The smaller the size of the precipitated particles reaching the solidification front the less is the tendency for lateral segregation. Unfortunately, the requirement of small sized precipitated particles together with their low velocity arrival at the primary solidification front is in conflict with the requirement for obtaining a solidification front approaching the ideal or substantially horizontal condition. The precipitated particles will become smaller with (i) increasing extent of supercooling, (ii) lower lead content of the alloy and (iii) with increasing rates of casting. Therefore, the solution to these conflicting requirements is somewhat of a compromise between obtaining a casting rate such as to promote a suitably flat solidification front and, at the same time, promote small sized precipitated particles.

In conventional continuous casting techniques the problem of controlling the minimum casting temperature isothermal so that it does not extend into the tundish is not encountered since it is not important other than when segregation in the liquid state is required to be avoided or controlled.

In methods according to the invention, after leaving the die 3, the alloy is in the condition in which it is part solid and part liquid and eventually after the secondary soldification front C the alloy is wholly solid and can be extracted by normal continuous casting extraction means.

As shown in FIGURE 3, the invention can be executed using a horizontal continuous casting technique. The minimum casting temperature isothermal A is confined wholly within the vertical section of the die 3, whereas the primary solidification front B is contained partially in the vertical and partially in the horizontal section of the die 3. The latter feature is most important to facilitate withdrawal of the cast product. The aluminium-lead alloy is shown being cast directly onto a backing strip of steel 6, moved horizontally under the die. In cases where direct casting of an alloy onto a backing strip, e.g. aluminium/ lead onto steel, gives rise to brittle intermetallic compound formation at the alloy/backing interface, the close control of the following parameters (see FIGURE 3) may 'be used to minimize the extent of such deleterious compound formation.

(1) V=casting speed.

(2) D=distance over which compound can form.

(3) T=D/V=time for compound formation.

(4) Geometry of vertical section of die, effectively reducing the value of D above.

It is an advantage of the present invention that since the lead and the aluminium exist as a homogeneous single liquid there is no problem of uniformly dispersing lead into liquid aluminium as might be associated with an alternative solution for casting this material for example by adding lead to molten aluminium. Moreover, oxide films forming on the surface of molten aluminium might interfere with the ingress of lead if this were added to molten aluminium. Furthermore, any turbulence may affect the dispersion of the lead in the aluminium which would then produce a material in which the two constituents are not evenly dispersed in the cast product. In the process according to the invention the lead distribution in the final product may be controlled by varying the temperature gradient occurring during the casting, for example, the temperature gradient between the primary solidification front B and the minimum casting temperature isothermal A A billet of aluminium-lead produced by a process according to the invention, may require further heat treatment to precipitate any lead remains as a non-equilibrium solid solution.

Due to the relatievly high-temperature necessary to carry out this invention a vacuum, or inert atmosphere may be required during melting the alloy and subsequent casting operations.

FIGURE 4 shows some examples of how the secondary phase of the alloy can be controlled as a non-uniform distribution in a primary phase by varying the geometry of the die. FIGURE 4A shows, for the example of an aluminium lead alloy, the dispersion of the secondary phase from a parallel die 7. The primary solidification front and minimum casting temperature isothermal are indicated.

The section underneath FIGURE 4A has the lead evenly dispersed and the reference N indicates the lead concentration obtained from using a parallel sided die. FIGURE 4B shows a stepped die 8 which gives the result shown in the underlying section of a lead concentration indicated by the region H which is higher than N and two outer regions where the lead concentration L is less than N. FIGURE 4C, shows a die 9,. of a convergent shape and this produces as shown in the section a region of concentration N and outer regions H of higher lead concentration. FIGURE 4D shows a combination of the effects of FIGURES 4B and C by using a stepped and tapered die 12. FIGURE 4E shows a die 113 having an entry portion 14 leading to two passages 15 and 16 and then opening into a parallel die portion. The die 13 as shown in the section gives a lead concentration L on the outside, two minor sections having a concentration of H and an inner region having a concentration the same as the two outer portions L N=the lead concentration for a parallel die, L=a lead concentration less than N and H=a lead concentration greater than N.

It is clear that by varying the positions of the minimum casting temperature isothermal (MCTI) and the primary solidification front (PSF) relative to the changes in section of the die, the extent as well asthe mode of distribution of the compositional variations can be controlled. It is to be understood that the principle of controlling the distribution of a secondary phase within a primary phase in a lateral plane is equally applicable to both vertical and horizontal continuous casting for example as shown in FIGURE 3.

The controlled bands of concentrations are carried out with a view to providing a range of bearing materials suitable for various applications and also with a View to the bonding of the cast material onto steel backing strip. An outside layer of reduced concentration of lead say L may assist the bonding of the cast material across the face to a metal backing; an increased content of lead say H, may be advantageous at parts of a journal bearing surface spaced from the edge.

To enable the casting of alloys exhibiting a secondary phase less dense than the primary phase e.g. leadaluminium alloys, with a controlled distribution of one phase within the other it is necessary to position the primary solidification front above the minimum casting temperature isothermal. This can be achieved by commencing to cast in a vertically upwards direction as shown in FIGURE 5 where a pressure head of metal is maintained to ensure continuous feeding of molten metal under pressure. FIG. 5 illustrates this concept and shows a substantial head of liquid alloy composition above the level at which secondary solidification phenomenon occurs in the die (here shown as having a cooling jacket about it). The height of said liquid alloy is indicated by the double-arrowed vertical line extending above the said level.

The invention while concerned with controlling or preventing segregation between two liquid phases is equally applicable to alloys in which a solid phase segregates through a continuous liquid phase.

Due to the temperatures involved in the casting, for example, of aluminium base alloys according to the invention being higher than normally encountered in cnventional continuously casting of these alloys, safety precautions may be required to overcome a potential explosion hazard. It may have to be ensured for example that should molten metal and coolant come into intimate contact accidentally that the rate of chemical reaction never exceeds, that which is likely to cause an explosion. Thus it is thought, that rapid thermal vapourisation of the metal or alloy being cast could be a potential explosion hazard.

What we claim as our invention and desire to secure by Letters Patent is:

1. A method of continuously casting alloys of the kind which on cooling exhibit a transition from a single homogeneous phase to a two-phase liquid region to be cast, comprising the steps of causing the alloy to flow from a feeder to a casting die, the commencement of movement being in a vertical direction, maintaining the temperature of the alloy in the feeder high enough for the alloy to remain as a homogeneous liquid phase in the feeder with a minimum casting temperature isothermal lying substantially within the upstream end of the casting die, cooling the metal at a rate such that the primary solidification temperature is within the die, and withdrawing the cast metal from the die when the temperature of the metal has reached the secondary solidification temperature.

2. The method defined in claim 1, in which the casting die restricts the area of metal laterally of the direction of casting substantially in relation to the area of the feeder.

3. A method of continuously casting alloys of the kind which on cooling exhibits a transition from a single homogeneous phase to a two-phase liquid region in which region one phase precipitates relative to the other, comprising the steps of causing the alloy to flow to a casting die, the commencement of movement being in a vertical direction; maintaining the temperature of the alloy high enough for the alloy to remain as a single homogeneous liquid phase with a minimum casting temperature isothermal lying substantially within the upstream end of a casting die; extracting heat from the alloy such that the minimum casting temperature isothermal and a primary solidification front are substantially normal to the direction of precipitate movement, maintaining the isothermal and the primary solidification front so disposed relative to one another that the rate of precipitation generation of one phase relative to the other is substantially matched by the rate at which the precipitate is arrested at the primary solidification front and withdrawing the cast metal from the die when the temperature of the metal has reached a secondary solidification temperature at which both said phases are solid.

4. A method as claimed in claim 1 in which the die is of sufiicient length in the direction of casting to enable heat extraction from the die to cause solidification of the alloy.

5. A method as claimed in claim 1 of casting an aluminium-lead alloy.

6. A method as claimed in claim 5 in which there is 20% by weight of lead and the feeder temperature is not less than 1100 C.

7. A method as claimed in claim 6 in which the temperature of the alloy is reduced by the order of 450 C. in the die.

8. A method as claimed in claim 5 in which there is 10% by weight of lead and the temperature in the feeder is at least 950 C.

9. A method as claimed in claim 8 in which the alloy is reduced by the order of 300 C. in the die.

10. A method as claimed in claim 1 in which the cast material leaves the die vertically.

11. A method as claimed in claim 1 in which the cast metal leaves the die horizontally.

12. A method of casting an alloy as claimed in claim 1 in which the metal is cast directly on to a metal backing adjacent the outlet from the die.

References Cited UNITED STATES PATENTS 635,054 10/1899 McAdams 164-122. 2,672,665 3/1954 Gardner et al. 164-283 X 2,715,252 8/1955 Schaefier et al. 164-283 X 3,354,935 11/1967 Mann 164-126X 3,410,331 11/1968 Miller et al. 164-51 FOREIGN PATENTS 598,409 5/1960 Canada.

WILLIAM J. STEPHENSON, Primary Examiner R. SPENCER ANNEAR, Assistant Examiner US. Cl. X.R. 164-122 

